The present invention relates to chippers and the structures which hold the chipping knives.
The principal use of wood apart from its use as fuel and as a structural material is as a source of fiber. If the wood fibers are to be separated from the other constituents of wood, principally lignin, the wood must be chemically treated. The chemicals used, such as caustic soda, can damage the wood fibers if the wood fibers are exposed to the chemicals too long. For this reason, wood which is to be treated to extract the lignin is first reduced to wood chips which have a uniform thickness. These uniform wood chips minimize the time during which the wood fibers are exposed to the chemicals or cooking liquor. The wood chip allows the cooking liquor to act on all sides rapidly and uniformly, separating the wood fibers from the lignin of the wood. Of course the cutting of the wood into chips necessarily breaks some fibers and broken fibers have less or no value. Therefore a chipper which produces uniform chips and converts a high fraction of the raw logs to wood chips is desired. The production of wood fiber is a commodity business where profit margins are thin, so small improvements in quality, or in cost of production are the main sources of increased profitability.
Wood chippers are extremely productive machines reducing perhaps 70 to 170 cords of wood to chips in one hour. This high throughput, combined with the natural contamination of dirt and sand, results in the cutting blades and the blade supports being worn away. The blade base which is positioned directly below the cutting blades has a chip facing surface which is particularly subject to abrasion. The wood chips are actually broken into chips by colliding with this surface of the base and thus considerable wear takes place on the blade base immediately below the supported blade. One known approach is to simply replace the blade bases when they become worn, however, this adds to the cost of producing the wood chips. Another approach is to apply a surface hardening such as by flame spraying. A further approach is to weld on a piece of wear resistant material to the surface of the blade base exposed to high wear. These approaches, while extending the life of the blade bases, are undesirably labor-intensive. U.S. Pat. No. 5,765,452 describes a known technique which is to arrange a changeable blade stopper between the blade base and the blade. However U.S. Pat. No. 5,765,452 discourages using this approach. What is needed is a blade base which has a chip facing surface which can be replaced with minimal overall cost.